1. Field of the Invention
The present invention relates to a crankcase structure of an internal combustion engine.
2. Background of the Invention
In an internal combustion engine, a crank shaft and a counter shaft are rotatably supported on a partitioning plane by being sandwiched between an upper crankcase and a lower crankcase of a crankcase configured to have upper and lower halves. A main shaft is rotatably supported by the upper crankcase. Output members such as an output sprocket or an output gear are fitted to an end portion of the counter shaft projecting outward from the crankcase, to drive a rear wheel via a chain, or to drive an output shaft through meshing of gears.
An example of such internal combustion engine is disclosed in Japanese Patent No. 2670109. In a crankcase of an internal combustion engine disclosed in Japanese Patent No. 2670109, opposed left and right walls rotatably support the main shaft and the counter shaft. A rear wheel drive sprocket is fitted to the left end of the counter shaft which penetrates through the left wall. A left bearing opening on the left wall, which rotatably supports the left end of the main shaft via a bearing, is also an opening for removing the main shaft and hence has a large inner diameter.
The left wall of the crankcase, on which the bearing opening of a large diameter for the main shaft is formed, includes a bearing opening in which the counter shaft is rotatably supported via the bearing. The left bearing opening for supporting the counter shaft is located near the rear wheel drive sprocket and hence is subject to a large load. Therefore, the counter shaft must be rotatably supported robustly with a large bearing. However, the left bearing opening for supporting the main shaft is formed adjacent to the left bearing opening for supporting the counter shaft on the left wall of the crankcase.
As described above, the left bearing opening for supporting the main shaft has a large inner diameter for removing the main shaft, and the left bearing opening for supporting the counter shaft formed adjacent thereto also has to be formed to have a large inner diameter for fitting the large bearing. Therefore, it is difficult to maintain the strength of the left wall of the crankcase in the periphery of the rear wheel drive sprocket, which is to be fitted to the counter shaft.
In the internal combustion engine having the output shaft which projects from the front and rear of the crankcase for driving a front wheel or a rear wheel, the output shaft penetrates through the crankcase. Hence, the internal combustion engine tends to be increased in weight and size because the thickness is increased in order to secure the strength of the crankcase.